Purification and determination of the primary structure of renin laid the foundation for determining the complete structure and function. Identification of the functional groups in the active site and demonstration of the sequence homology of renin and aspartyl proteins have been the most important steps to date in characterizing the general properties of renin. The demonstration of a long subsite structure in the active site of renin and other aspartyl proteases was another important step. The lack of immuno-crossreactivity and substrate specificity between human and non-primate renin suggests that the active site of human renin is unique. This has been clearly demonstrated by the synthesis of an inhibitor peptide specific for human renin.